Technology update

Sep 28, 2017

Quantum-dot superlattices allow for precise band-gap engineering

Ordered arrays of semiconducting quantum dots (QDs) can be used in a wide range of optolectronic applications, from light-emitting devices, information displays, photovoltaics, biosensors, nanolasers and photodetectors. However, to realize such applications, researchers need to be able to pattern or print QDs into predetermined locations on a solid-state substrate with nanometre-scale resolution, something that has proved difficult. A team of researchers at the University of California, Santa Barbara, and Rice University in Texas, has now used a focused electron beam to fabricate a large-scale quantum-dot superlattice on an atomically thin sheet of 2D semiconducting molybdenum disulphide (MoS2). Each QD in the lattice is positioned at a precise location on the MoS2 and has a pre-determined size. The QDs can also be placed less than 4 nm apart to make an artificial crystal whose band gap can be tuned at will, from 1.8 to 1.4 eV.

This is the first time that researchers have succeeded in creating large-area 2D superlattices on an atomically thin material like MoS2, in which both the size and position of QDs can be controlled on the nanoscale. Each quantum dot acts as a quantum well in which electrons and holes can recombine to generate light. What is more, all the dots in the lattice are close enough to interact with one other, so that they form an artificial crystal.

The team, led by Kaustav Banerjee at UCSB, irradiated a monolayer of MoS2 with a 30 kV electron beam at room temperature using a scanning electron microscope (SEM) equipped with a Nanometer Pattern Generation System (NPGS). The beam has a diameter of 2 nm and was made to spot the MoS2 surface point-by-point. The beam induces a local 2H to 1T phase change in the MoS2 at these points, that is, it transforms the material at these points from being semiconducting to metallic.

Varying the size of the dots and their pitch

“By changing the irradiation dose of the electron beam, we can vary the size of the dots and the spacing between them (their pitch),” explains Banerjee. “This means that we can control their band gap from 1.81 eV (for pristine monolayer MoS2) to 1.42 eV. Using this technique, we can engineer the bandgap to match the final application.”

Until now, such tuneable-gap QD superlattices were fabricated using bottom-up techniques in which atoms automatically combine to form a macro-sized object. However, it is difficult to design precise lattice structures using such methods. “Our new approach is a photoresist-free, top-down method to create large-area QD arrays with nanometre-scale spatial density,” says Banerjee.

Towards a new generation of light-emitting devices

To prove that their technique works, the researchers produced an image showing “UCSB” spelled out in a grid of QDs with different areas of the university’s logo lighting up at different wavelengths when irradiated. The wider bandgap regions produce shorter wavelengths of light while the narrower bandgap regions longer wavelengths.

“Being able to tune bandgaps in this way could help in the creation of a new generation of light-emitting devices for photonics applications, adds Banerjee, “and we are now busy experimenting with these 2D superlattices to demonstrate such devices.”